The effects of neonatal hydrocephalus on the levels of tyrosine, tryptophan, 5-hydroxyindoleacetic acid (5-HIAA), and homovanillic acid (HVA) in CSF were determined by high-performance liquid chromatography (HPLC) with fluorometric detection in normal and chronically hydrocephalic rabbits. The hydrocephalic rabbits showed a highly significant increase in both the serotonin metabolite 5-HIAA and the dopamine metabolite HVA. There were no significant effects of the hydrocephalus on either tyrosine or tryptophan levels. There was a significant positive correlation between the intracranial pressure (ICP) and the increase in 5-HIAA and HVA, but not with the two precursor amino acids. There was a significant decrease in these amino acid precursors with age in both groups. A trend towards higher levels of 5-HIAA and HVA in older rabbits was also evident, however this change was not to the degree found in the hydrocephalics. These data indicate that increased ICP affects the mechanism of removal of 5-HIAA and HVA from the cerebrospinal fluid.